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Quantum Cryptography: 100km Barrier Broken
jdfox writes "Toshiba Research Europe have just demonstrated quantum crypto over 100km fibre links. Sounds like there's still a fair bit of work to be done before it leaves the lab, but it's amazing that they've got as far as they have. There's another article about it, though still not much technical detail, here on the BBC and here on The Register."
I'm not sure whether this would work or not (since you reading the photon is what changes its spec... you'd be reading the new version of the photon, I'd think and would need the original key to put it back the way it was...)
But without pretty spiffy splicing techniques, how long do you think it would take to get that repeater inserted into a fibre link? When I was in college, a friend of mine got a job fusing splices in fibre optic lines with a special machine, and it still took him several minutes per splice once he got good with it. The other end is going to know something's up when the fibre goes dark for more than a few ms...
If I have been able to see further than others, it is because I bought a pair of binoculars.
Yes. However, quantum mechanics is an extremely well-established theory.
As a physicist, I'm reluctant to call anything a fact. However, just because I cannot prove that (say) gravity won't cease to exist tomorrow morning, doesn't mean I live under the constant fear that this might in fact happen. Much in the same way, I'm confident that nothing is wrong with quantum mechanics.
I was re-reading the Fabric of Reality (David Deutsch) ... which essentially covers Quantum interference / computing (with the arguement that Quantum computing is a result of multiple universes coming together and interfereing with one another) ... In any case this may be a little bit off topic ... but the book echos 'The Matrix Reloaded' in many ways ... Deutsch describes an 'Oracle' who knows everything ... A Virtual Reality machine that interfaces with the brain (even a picture that looks like something out of the Matrix) ... a multiverse (worlds within worlds etc..) ... and a Universal Virtual Reality Generator that can essentially recreate the environment we live in ... in real time. This book pre-dates the original Matrix by a year.
If there are several photons in the same arbitrary state, you can by measuring the qubits in different basis each time, come up with an approximation to the actual quantum state. If there are a 1000 of these photons, then basically we aren't gaining anything by having our information in Quantum form. So you want to avoid sending many duplicate photons for many of the states that you are sending.
(This may be inaccurate as I'm recalling it from what I read in Simon Singh's "The Code Book", but I hope it explains the point.)
The idea is that you can measure the photons with only partial accuracy, and according to the setting of the measuring instrument. For example, if sending a photon in state Y, the measurement does not yield: "The photon was in state Y", but instead "The photon was probably in state X but maybe in state Y or Z, and not in state W.". Another measurement configuration could yield: "The photon was probably in state Y but maybe in state X or W, and not in state Z."
The "hacker" does not know the measurement configuration at the receiver and may try some arbitrary configuration of his own.
The problem is, when receiving the measurement result, for example that the photon was probably in state X, trying to retransmit it as X may be picked up as inconsistent at the real receiver's.
The measurement configuration itself for each bit can be agreed upon by a negotiation stage where a bitstream is sent accross random configurations of both the sender and receiver and then publically agreeing which bits of the sequence to use (knowing they have matching configurations, not letting a "hacker" enough information to know what configurations those are - leaving him with impossible guesswork).